1. Field of the Invention
The present invention relates to a valve casing for use in a butterfly valve and, more particularly, to a novel structure of a valve casing formed by press working of metal plates.
2. Description of the Prior Art
A valve casing having been conventionally manufactured by casting a metal material such as cast iron or non-ferrous metal and machining a cast product, but recently as a result of advances in sheet pressing, the valve casing is also manufactured by a method wherein a iron or non-ferrous metal plate is formed into a valve casing by press working and a machining process thereof is either omitted or less applied. By such a method, a large amount of valve casings are now efficiently mass-produced with their dimensional accuracy assured, thereby resulting in considerably savings in the manufacturing costs thereof in comparison with the conventional method by casting and machining.
Furthermore, since the weight of a valve manufactured by the aforesaid recent method is much lighter, an advantage can be enjoyed when installing it on a pipe line, and since loads applied to the pipe line during the operation of the valve are decreased, a further advantage is enjoyed from the standpoint of maintenance.
Notwithstanding such advantages, a valve casing used in butterfly valves produced by press working has several disadvantages mainly due to poor thickness which are not the case for a valve produced by conventional casting.
For example, since the thickness of a molded valve casing is relatively poor, a stress concentration is applied to seal ring receiving portions when joining the molded valve casing to an adjacent pipe, resulting in deformation or breakage.
There is another problem that a seal ring gets out of place or broken, bringing about imperfect sealing during operation.
A further problem exists in that heat energy is lost when the temperature of a fluid passing through the passage is held while outside air temperature is low. To the contrary, when the temperature of a fluid is low while that of the outside air is high, there is the possibility that such problems as freezing, pipe breakage, etc. arise in the process of heat transference, although these problems are not peculiar to the valve casings produced by press working but are common in every valve casting.